Lithographic Performance Research Articles

Chemically Amplified Molecular Glass Photoresist Regulated by 2-Aminoanthracene Additive for Electron Beam Lithography and Extreme Ultraviolet Lithography.

2-Aminoanthracene was used as a nucleophilic additive in a molecular glass photoresist, bisphenol A derivative (BPA-6-epoxy), to improve advanced lithography performance. The effect of 2-aminoanthracene on BPA-6-epoxy was studied by electron beam lithography (EBL) and extreme ultraviolet lithography (EUVL). The result indicates that the additive can optimize the pattern outline by regulating epoxy cross-linking reaction, avoiding photoresist footing effectively in EBL. The EUVL result demonstrates that 2-aminoanthracene can significantly reduce line width roughness (LWR) for HP (Half-Pitch) 25 nm (from 4.9 to 3.8 nm) and HP 22 nm (from 6.9 to 3.0 nm). The power spectrum density (PSD) curve further confirms the reduction of roughness at medium and high frequency for HP 25 nm and the whole range of frequency for HP 22 nm, respectively. The study offers useful guidelines to improve the roughness of a chemically amplified molecular glass photoresist with epoxy groups for electron beam lithography and extreme ultraviolet lithography.

Effective Optimization Strategy for Electron Beam Lithography of Molecular Glass Negative Photoresist

AbstractAs the crucial dimension (CD) of logic circuits continues to shrink, the photoresist metrics, including resolution, line edge roughness, and sensitivity, are faced with significant challenges. Photoresists are indispensable in the integrated circuit manufacturing industry, and specifically in achieving smaller critical dimensions. In this study, the effects of two categories of photosensitive compounds on lithography performance are explored, through a series of sulfonium salt‐based photoacid generators (PAGs) with diverse reactivity and photodegradable nucleophiles (PDNs) with varying nucleophilicity. The detailed characterization and exposure experiments suggest that the reactive alterations of different PAGs are mostly associated with the amount of phenyl composed of cations in PAGs. The “PDN first, PAG second” strategy, which employs a combination of low reactivity PAG and high reactivity PDN and involves PDN decomposition first and PAG decomposition second in the electron beam lithography process, achieves high sensitivity (100–270 µC cm−2), high resolution (25 nm 1:1 line/space, L/S), and low line edge roughness (LER ≤ 3.3 nm) stripes. This approach outperforms conventional formulations and may provide a potentially effective and useful strategy to improve electron beam photoresists.

Effective multi-objective inverse lithography technology at full-field and full-chip levels with a hybrid dynamic priority algorithm.

Inverse lithography technology (ILT), such as source mask optimization (SMO), is used to improve lithography performance. Usually, a single objective cost function is selected in ILT, and an optimal structure for one field point is achieved. The optimal structure is not the case for other images at full field points where the aberrations of the lithography system are different, even in high-quality lithography tools. The optimal structure that must match the high-performance images at the full field is urgently required for extreme ultraviolet lithography (EUVL). In contrast, multi-objective optimization algorithms (MOAs) limit the application of multi-objective ILT. Assigning target priority is incomplete in current MOAs, which results in the over-optimization of some targets and under-optimization of others. In this study, multi-objective ILT and a hybrid dynamic priority (HDP) algorithm were investigated and developed. High-performance images with high fidelity and high uniformity were obtained at multi-field and multi-clip areas across the die. A hybrid criterion was developed for the completion and reasonable prioritization of each target to ensure sufficient improvement. Compared to the current MOAs, the uniformity of images at full-field points was improved by up to 31.1% by the HDP algorithm in the case of multi-field wavefront error-aware SMO. The multi-clip source optimization (SO) problem showed the universality of the HDP algorithm to deal with different ILT problems. It acquired higher imaging uniformity than existing MOAs, which indicated that the HDP is more qualified for multi-objective ILT optimization than existing MOAs.

Inverse Lithography Source Optimization via Particle Swarm Optimization and Genetic Combined Algorithm

Inverse lithography technologies (ILTs) are critical for improving the imaging performance of lithography in advanced technology nodes. Pixel-based source optimization (SO), as an efficient part of ILTs, can be implemented <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">via</i> heuristic approaches to achieve high-performance lithographic imaging. In this paper, a SO approach based on a combination of the particle-swarm optimization and genetic algorithms (PSO–GA) is proposed to determine the optimal intensity distribution of the source <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">via</i> iterations. The pixelated source can be decoded into the optimized variables of the merit functions in the SO model. The proposed PSO–GA algorithm, as a high-efficiency hybrid algorithm, can transform the discrete SO problem into the optimal search solution for the merit function, thereby inversely enhancing the lithographic-imaging performance. In the forward-imaging model in the lithography, the extraction of the mask's effective diffraction spectrum is implemented to calculate the layout of resist patterns. The simulation results highlight the superior performance of the proposed approach in achieving pixelated SO over the traditional GA and PSO algorithm in terms of convergence capacity.

Enhancement of pattern quality in maskless plasmonic lithography via spatial loss modulation

Plasmonic lithography, which uses the evanescent electromagnetic (EM) fields to generate image beyond the diffraction limit, has been successfully demonstrated as an alternative lithographic technology for creating sub-10 nm patterns. However, the obtained photoresist pattern contour in general exhibits a very poor fidelity due to the near-field optical proximity effect (OPE), which is far below the minimum requirement for nanofabrication. Understanding the near-field OPE formation mechanism is important to minimize its impact on nanodevice fabrication and improve its lithographic performance. In this work, a point-spread function (PSF) generated by a plasmonic bowtie-shaped nanoaperture (BNA) is employed to quantify the photon-beam deposited energy in the near-field patterning process. The achievable resolution of plasmonic lithography has successfully been enhanced to approximately 4 nm with numerical simulations. A field enhancement factor (F) as a function of gap size is defined to quantitatively evaluate the strong near-field enhancement effect excited by a plasmonic BNA, which also reveals that the high enhancement of the evanescent field is due to the strong resonant coupling between the plasmonic waveguide and the surface plasmon waves (SPWs). However, based on an investigation of the physical origin of the near-field OPE, and the theoretical calculations and simulation results indicate that the evanescent-field-induced rapid loss of high-k information is one of the main optical contributors to the near-field OPE. Furthermore, an analytic formula is introduced to quantitatively analyze the effect of the rapidly decaying feature of the evanescent field on the final exposure pattern profile. Notably, a fast and effective optimization method based on the compensation principle of the exposure dose is proposed to reduce the pattern distortion by modulating the exposure map with dose leveling. The proposed pattern quality enhancement method can open new possibilities in the manufacture of nanostructures with ultrahigh pattern quality via plasmonic lithography, which would find potentially promising applications in high density optical storage, biosensors, and plasmonic nanofocusing.

A Single-Component Molecular Glass Resist Based on Tetraphenylsilane Derivatives for Electron Beam Lithography.

A novel molecular glass (TPSiS) with photoacid generator (sulfonium salt group) binding to tetraphenylsilane derivatives was synthesized and characterized. The physical properties such as solubility, film-forming ability, and thermal stability of TPSiS were examined to assess the suitability for application as a candidate for photoresist materials. The sulfonium salt unit underwent photolysis to effectively generate photoacid on UV irradiation, which catalyzed the deprotection of the t-butyloxycarbonyl groups. It demonstrates that the TPSiS can be used as a 'single-component' molecular resist without any additives. The lithographic performance of the TPSiS resist was evaluated by electron beam lithography. The TPSiS resist can resolve 25 nm dense line/space patterns and 16 nm L/4S semidense line/space patterns at a dose of 45 and 85 μC/cm2 for negative-tone development (NTD). The etching selectivity of the TPSiS resist to Si substrate is 8.6 under SF6/O2 plasma, indicating a potential application. Contrast analysis suggests that the significant solubility switch within a narrow exposure dose range (18-47 μC/cm2) by NTD is favorable for high-resolution patterns. This study supplies useful guidelines for the optimization and development of single-component molecular glass resists with high lithographic performance.

Synthesis of Acrylate Dual-Tone Resists and the Effect of Their Molecular Weight on Lithography Performance and Mechanism: An Investigation.

Acrylate photoresists have gained considerable attention in recent years owing to their high resolution, high sensitivity, and versality. In this work, a series of thermally stable copolymers are synthesized by introducing an isobornyl group, and well characterized using Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectra (1H-NMR). The effects of polymerization conditions on the molecular weight and their further influence on lithography are explored. By analyzing the thermal properties, film-forming capabilities, and the patterning behavior of these copolymers, a direct correlation between lithography performance and polymerization conditions is established via the molecular weight. In addition, the baking temperature of lithography is also optimized by atomic force microscopy (AFM), after which a line resolution of 0.1 μm is observed under the exposure of a 248 nm UV light and electron beam. Notably, our synthesized photoresist displays dual-tone resist characteristics when different developers are applied, and the reaction mechanism of acid-catalyzed hydrolysis is finally proposed by comparing the structural changes before and after exposure.

Generation of inverse assist features using generative adversarial networks

Adding the assist feature in the optical proximity correction (OPC) procedure is crucial for improving the process window. The rule-based assist feature placement is very fast; however, the resultant process window still requires improvement. The model-based assist feature placement presents a better process window, but its iterative nature makes it extremely time-consuming. The assist features (AFs) that are optimized by using the inverse lithography technique perform best in terms of lithographic metrics such as the Depth of Focus, Mask Error Enhancement Factor, and Normalized Image Log Slope; these features can be added to the objective function. However, the inverse lithography technique is a pixel-level correction method, which is even more time-consuming. Consequently, it is typically used to provide guidance for rule-based AF placement. In this study, we design novel generative adversarial networks (GANs) that can generate AFs to increase the speed of assist feature placement while maintaining relatively high lithographic performance using inverse lithography techniques. The results demonstrate that the proposed generative adversarial network methods are three orders of magnitude faster than the inverse lithography techniques. Additionally, they exhibit lower edge placement errors and process variation bands when compared to the inverse lithography techniques.

Curvilinear mask optimization with refined generative adversarial nets

Inverse lithography technology (ILT) can optimize the mask to gain the best process window and image quality when the design dimension shrinks. However, as a pixel level correction method, ILT is very time-consuming. In order to make the ILT method useful in real mask fabrication, the runtime of ILT-based optical proximity correction mask must evidently decrease while keeping the good lithographic metric performance. Our study proposes a framework to obtain the curvilinear ILT mask with generative adversarial network (GAN). It is subsequently refined with the traditional ILT to exclude unexpected outliers generated by the GAN method. We design conditional GAN, reverse GAN (RGAN), and high discretion GAN (HDGAN) to generate curvilinear ILT mask. Their runtime and the performance are compared. Compared with the CILT method, the speed of GAN type methods with the afterward refinement is increased by an order of magnitude. The RGAN has a better performance in edge placement error and process variation band evaluation, and HDGAN has a better performance in the mask error enhancement factor evaluation. The designed RGAN and HDGAN are promising in actual application to generate the curvilinear mask. They can evidently decrease the runtime and have better lithographic metric performance.

Shape-based proximity effect correction method for throughput, fidelity, and contrast enhancement of electron-beam writer

Conventional shape-based proximity effect correction (PEC) methods can only improve pattern fidelity and present a risk to correction effectiveness under severe resist-contour distortions for electron-beam writers. Herein, a shape-based PEC method for effectively improving both the pattern fidelity and energy slope without the risk in realizing high throughput, fidelity, and contrast is presented. An extra-framed pattern added to the target pattern is optimized to achieve better trade-off between contrast improvement and writing-time reduction. The proposed method demonstrated significant improvements in lithographic performance compared with the conventional and dose-based PEC methods for high-numerical-aperture extreme-ultraviolet mask and aggressive patterns.

Towards environmentally friendly processing of ionic liquid-based photoresists with a boosted lithography performance

The development of a sustainable process for producing high-performance photoresists is desirable yet challenging.

Dependence of Dissolution Kinetics of Main-Chain Scission Type Resists on Molecular Weight

The dissolution kinetics and molecular weight in main-chain scission type resists such as poly(methyl methacrylate) (PMMA) and ZEP520A were investigated using a quartz crystal microbalance (QCM) method and a Gel Permeation Chromatography (GPC) to clarify the effects of the molecular weight after polymer degradation on the dissolution kinetics. G-values of main chain scissions in PMMA and ZEP520A are estimated using GPC measurement. The G-value of ZEP520A showed higher G-values than that of PMMA. There are three molecular weight regions in PMMA, whereas there are only two molecular weight regions in ZEP 520A. The dissolution behavior depends on molecular weight after polymer degradation in both PMMA and ZEP 520A. These results indicate that ZEP520A is better to be better lithographic performance.

Controlled Sequence Photoresists from Polypeptoids

Low stochastics, high sensitivity photoresists remain a goal for lithography. Here we present a study of a polymer system, polypeptoids, using a synthetic strategy borrowed from the biomedical community to make improvements to these resist characteristics through control of polymer sequence. We describe peptoid polymers that possess identical molecular size, composition and sequence with higher molecular uniformity than possible by conventional synthetic techniques. We report the results of exposure of these materials to DUV radiation and the chemical changes that occur. In addition, we highlight unexpected observations of the role of sequence on lithographic performance.

Effect of Alternative Developer Solutions on EUVL Patterning

This work explores the application of alternative developer solutions (“developers”) with the aim of understanding their potential effectivity in the reduction of resist-based stochastic pattern defects (or “stochastic defects”) in extreme ultraviolet lithography (EUVL). Specifically, the application of a quaternary ammonium hydroxide type aqueous developer; ethyltrimethylammonium hydroxide (ETMAH) in comparison to the industry de facto standard aqueous alkali tetramethylammonium hydroxide (TMAH) developer was investigated. Focusing on EUV exposed contact hole (CH) patterns on a typical chemically amplified resist (CAR), the effect of these developers on stochastic defects were assessed. As a result, patterning investigations showed that the lithographic performance of the CAR developed in ETMAH is comparable to the those obtained with TMAH. In situ resist dissolution analysis using the high-speed atomic force microscope (HS-AFM) confirms this as it showed that the rate of CH formation during resist dissolution in both developers are relatively the same. Moreover, it was also understood that compared to the commonly used alkali developer concentration of 0.26N, a lower ETMAH developer concentration of 0.20N resulted in stochastic defect margin improvement, while maintaining lithographic performance. In situ resist dissolution analysis showed an obvious slowing down of CH pattern formation rate at 0.20N concentration, suggesting the possibility that of over-dissolution at the higher concentration condition, translating to an increase in merging CH defects. The results from this study show the advantages of further pursuing optimal developers for EUVL. This is especially significant as these findings indicate how optimal developers mitigate resist-based stochastic defects while maintaining lithographic performance.

Extend 0.33 NA extreme ultraviolet single patterning to pitch 28-nm metal design by low-n mask

Extending 0.33 NA extreme ultraviolet single patterning to 28-nm pitch becomes challenging in stochastic defectivity, which demands high-contrast lithographic images. The low-n attenuated phase-shift mask (attPSM) can provide superior solutions for individual pitches by mitigating mask three-dimensional effects. The simulation and experiment results have shown substantial imaging improvements: higher depth of focus at similar normalized image log slope and smaller telecentricity error values than the best binary mask configuration. In this work, the exploration of low-n attPSM patterning opportunity for pitch 28-nm metal design is investigated. Using generic building block features, the lithographic performance of the low-n attPSM is compared with the standard binary Ta-based absorber mask. In addition, the impact of mask tone (bright field (BF) versus dark field) on the pattern fidelity and process window is evaluated both by simulations and experiments. The results indicate that BF low-n attPSM provides the best patterning performance. Consequently, the BF low-n attPSM patterning performance is assessed with an actual imec N3 pitch 28-nm random logic metal design. The wafer data indicate BF low-n attPSM enables good patterning fidelity, as well as good overall process window with high exposure latitude (∼20 % ).

Photoresists based on bisphenol A derivatives with tert-butyl ester groups for electron beam lithography

Four branched bisphenol A derivatives decorated with tert-butyl ester group (BPA-BU-n, n = 1–4) were synthesized. The number of BU groups was controlled precisely by selective hydrogenation. The good thermal resistance and film-forming performance suggested that BPA-BU-1 and BPA-BU-2 are candidates for photoresist materials. They were used as positive-tone photoresists by mixing with a proportionate photo-acid generator and base. Both BPA-BU-1 and BPA-BU-2 resists can obtain 40 nm L/S patterns. BPA-BU-1 resist exhibits higher sensitivity and contrast than that of BPA-BU-2 resist for their different substituted positions of acid labile groups in the molecule. All the results demonstrated that protected ratio and position had a significant effect on the patterning ability of BPA-BUs resists. This study will help to understand the relationship between molecular structures and lithographic performance, and provide useful guidelines for designing molecular resists.

Modular Synthesis of Phthalaldehyde Derivatives Enabling Access to Photoacid Generator-Bound Self-Immolative Polymer Resists with Next-Generation Photolithographic Properties.

The resolution, line edge roughness, and sensitivity (RLS) trade-off has fundamentally limited the lithographic performance of chemically amplified resists. Production of next-generation transistors using extreme ultraviolet (EUV) lithography depends on a solution to this problem. A resist that simultaneously increases the effective reaction radius of its photogenerated acids while limiting their diffusion radius should provide an elegant solution to the RLS barrier. Here, we describe a generalized synthetic approach to phthalaldehyde derivatives using sulfur(VI) fluoride exchange click chemistry that dramatically expands usable chemical space by enabling virtually any non-ionic photoacid generator (PAG) to be tethered to phthalaldehyde. The resulting polymers represent the first ever PAG-tethered self-immolative resists in an architecture that simultaneously displays high contrast, extraordinary sensitivity, and low roughness under EUV exposure. We believe this class of resists will ultimately enable researchers to overcome the RLS trade-off.

Synthesis of N‐Substituted Maleimides and Poly(styrene‐co‐N‐maleimide) Copolymers and Their Potential Application as Photoresists

AbstractPoly(styrene‐co‐N‐maleimide) copolymers bearing tert‐butoxycarbonyl (t‐BOC)‐protected amine groups attached to side chains of varying lengths are synthesized via activators regenerated by electron transfer atom transfer radical polymerization (ARGET‐ATRP) and investigated from the perspective of photoresist applications. The length of the alkyl substituents enables control of thermal properties as well as hydrophobicity, which are critically important for resist processing. Removal of the acid labile t‐BOC group during deep‐UV (DUV)exposure shifts solubility in the exposed areas and well‐defined line space patterns of 1 µm are obtained for the selected copolymers. The correlation between glass transition temperature (Tg) and solubility contrast determines the lithographic performance where the copolymers with shorter alkyl chains exhibit promising results.

Molecular Glass Resists Based on Tetraphenylsilane Derivatives: Effect of Protecting Ratios on Advanced Lithography

A series of t-butyloxycarbonyl (t-Boc) protected tetraphenylsilane derivatives (TPSi-Bocx, x = 60, 70, 85, 100%) were synthesized and used as resist materials to investigate the effect of t-Boc protecting ratio on advanced lithography. The physical properties such as solubility, film-forming ability, and thermal stability of TPSi-Bocx were examined to assess the suitability for application as candidates for positive-tone molecular glass resist materials. The effects of t-Boc protecting ratio had been studied in detail by electron beam lithography. The results suggest that the TPSi-Bocx resist with different t-Boc protecting ratios exhibit a significant change in contrast, pattern blur, and the density of bridge defect. The TPSi-Boc70% resist achieves the most excellent patterning capability. The extreme ultraviolet (EUV) lithography performance on TPSi-Boc70% was evaluated by using the soft X-ray interference lithography. The results demonstrate that the TPSi-Boc70% resist can achieve excellent patterning capability down to 20 nm isolated lines at 8.7 mJ/cm2 and 25 nm dense lines at 14.5 mJ/cm2. This study will help us to understand the relationship between the t-Boc protecting ratio and the patterning ability and supply useful guidelines for designing molecular resists.

The Recent Progress of Lithography Machine and the State-of-art Facilities

With the rapid development of industrial intelligent manufacturing and electronic information technology, the importance of integrated circuits has grown fast. Photolithography, as the core technology of integrated circuit industry, has become a key research target for researchers all over the world. In this paper, we provide a brief introduction to photolithography as well as an outlook on the future development direction. Firstly, the key metric of lithography system, which is resolution, and how it relates to lithographic performance is analyzed. Secondly, some exposure methods developed on UV and DUV light sources are discussed, which are commonly used in the industry nowadays. Subsequently, this paper presents the structure and performance of some representative lithography equipment. Then, some summarizations are completed about the most recent advances in EUV lithography and high NA lithography. Finally, we examine the limitations of current lithography and forecast the future of lithography. The goal of this paper is to provide a guide on lithography equipment, particularly the most advanced products available nowadays. Additionally, some potential challenges that photolithography may face in its future development are highlighted, and some perspectives on how photolithography will evolve over the next decade are provided. These results shed light on guiding the future development direction of lithography machine as well as ways to push Moore’s law even further.